Ceiling luminaires of the type widely used in commercial installations normally consist of a fixture or housing into which one or more lamps may be mounted, along with appropriate electrical connections. The lamps may of a variety of different types, such as incandescent, high-intensity gas discharge, or fluorescent lamps. For commercial installations, elongated fluorescent lamp tubes currently are the most popular. Frequently, some form of a shielding medium is used in the light-emitting opening or aperture of the fixture to direct the light rays into preferred directions. This is done for a number of purposes, such as reducing glare, to hide the appearance of the lamps, or to direct the light rays toward objects to be lighted. Consequently, there may be aesthetic benefits, the creation of increased visibility, and an associated reduction in the total amount of energy consumed by the lighting system.
Shielding media also may of a variety of different forms; but such media usually are in one or the other of two categories, namely, reflecting or transmitting. A popular example of a reflecting medium is a louver assembly. Louver assemblies typically consist of vertical or slightly curved blades arranged in a generally vertical orientation in the opening of the luminaire housing. These blades are arranged in a pattern, generally in the form of longitudinal and transverse louvers, to form a number of apertures through which the light passes. The louver blades are typically formed of metal or metalized plastic; so that light rays striking a louver blade are reflected in a generally downward direction.
Louver assemblies have become very popular because of their ability to significantly reduce or eliminate glare, which otherwise would be seen by a viewer looking at the lighting fixture at angles close to horizontal, for example, from 50.degree. to 90.degree. from the downward vertical. Louvers formed from metal or employing metalized plastic blades which have a slight curvature to the blade surface are widely used. These louvers intercept light rays traveling at high angles from nadir, and redirect such light rays to lower angles Rays which are emitted from the lamps within the fixture at lower vicinity of the lighting fixture, rather than traveling across the space to be viewed as glare. Rays which are emitted from the lamp in the fixture at low angles (near vertical) pass through the louver apertures uninterrupted.
When louver assemblies are constructed from highly reflective material, they are generally efficient, that is, the total lumen output of a fixture equipped with such a louver assembly as a percentage of the total lumens generated by the lamp is fairly high. Louver assemblies, however, have three major disadvantages. One disadvantage relates to the appearance of louvered fixtures in general. A second disadvantage is connected with the width of the overall light pattern produced by the louvers and their associated luminous efficiency. A third disadvantage concerns the appearance of the actual louver blades in the louver assembly.
The first of the disadvantages of louver assembly concerns the "shielding angle" of such assemblies. The shielding angle is determined by the closeness of the spacing of the louver blades and their vertical width or depth. If a viewer looks up toward the lighting fixture such that his line of sight makes an angle with horizontal which is less than the shielding angle, an attractive appearance of the fixture is presented to the viewer. If a viewer however, looks up at the lighting fixture such that his line or sight makes a greater angle to the horizontal than the shielding angle, the viewer then has a clear view of the bright light source and the various electrical and hardware items located inside the fixture. From an architectural standpoint, this is undesirable and it is aesthetically unattractive. If a viewer happens to look up into a fixture, the viewer also is subjected to glare from the direct observation of the lamps within the fixture.
This appearance problem of louver assemblies can be improved somewhat by increasing the shielding angle. This may be accomplished in one of two ways, the first of which is to reduce the distance of separation between the longitudinal and transverse louver blades. Doing this, however, creates another disadvantage. Densely spaced blades cause a greater proportion of the light rays to be intercepted, rather than passed freely through the apertures in the louver assembly. A typical blade reflectance of 85 percent thus creates an absorption of 15 percent of any light rays striking the blade An increase of the total wattage of the lighting system then is needed to maintain a required level of work surface illumination. Because of widespread current concerns of energy conservation, the trend within the lighting industry is to reverse this effect by using larger aperture sizes in a louver assembly for the lighting of general spaces, rather than using smaller aperture sizes.
The shielding angle also can be increased by extending the vertical depth of the louver blades in the louver assembly. The same problem of causing a greater proportion of light rays to be intercepted, rather than passed freely through the apertures exists when this approach is taken. In addition, another problem is presented from an architectural or structural standpoint as much as the deeper blades require a fixture having greater vertical depth; so that the blades either extend below the ceiling line or greater space above the ceiling line is required for the fixture.
When closer louver blade spacing is used to increase the shielding angle, another problem is introduced inasmuch as a greater proportion of blade tops, or horizontal upper areas of the blades, exists. These blade tops intercept light and do not redirect the light into usable directions. Consequently, the luminous efficiency of the fixture is reduced; and the power necessary to achieve any given lighting level must be increased.
The second problem or disadvantage of louvered light fixtures concerns the width of the overall light pattern produced by the louvers, and the associated efficiency mentioned above. The direction of any ray of light from the light source within the fixture is affected by the angle of the louver blade at the point of interception.
The blade depth is maintained constant (typically, in a range from 0.5" to 4"), a decrease in the radius of curvature of the blade at any or all points along its profile causes the width or the horizontal top of the louver blade to increase. This radius decrease lowers the angle of the reflected ray, but an increased top surface area of the louver blades of the assembly decreases the luminous efficiency of the fixture. Consequently, a louver assembly designed to be highly concentrating, that is, producing a strong downward disposition of light rays close to nadir, will have low efficiency. As mentioned above, most louver assemblies have blades which run both longitudinally and transversely of the fluorescent tube in the fixture to form square or rectangular openings or apertures through which the light passes. Consequently, the effect of an increased top width of the louver blades is multiplicative, and rapidly reduces the open top area of the apertures. This results in a relatively low efficiency from light concentrating louvers. Frequently, such louvers absorb more light than is emitted through the fixture, that is, fixture efficiency is less than fifty percent (50 %).
In spite of the very low efficiency and associated energy penalty, concentrating louver assemblies are widely used in commercial installations. This is, in large part, a result of the widespread use of electronic devices which incorporate video display units (VDU's). Lighting for areas which contain such VDU's typically is recommended to be obtained from highly concentrating fixtures to minimize reflection of light from the fixtures to the VDU screens and back into the eyes of the operators. For example a fixture which produces medium spread or wide spreading of the rays of light from the fixture, allows light to fall on the VDU screen and reflect into the eyes of the operator, who may need to operate such a VDU terminal for prolonged periods of time. This is considered a serious problem by engineers, architects, management health, union and government authorities. Concentrating fixtures which produce rays clustered close to nadir, either do not have rays which strike the VDU screen at all, or, when they do, they are reflected in a generally downward direction away from the operator's eyes. This produces a visibility increase and a freedom from reflected glare, with an associated reduction in visual error and eye fatigue.
A third disadvantage of louver blade assemblies concerns the appearance of reflected light in the louver blades themselves. If the louver blades are manufactured with highly specular surfaces (to provide maximum efficiency), there is an appearance within the louver when it is viewed from certain angles (typically, close to the shielding angle), of very dark and very bright areas in close proximity. Even on an individual louver blade, part of the blade will be lighted brightly by the reflected image of a lamp in the fixture, while another part, because of an angular change, is not positioned to reflect an image and appears nearly black. This produces a displeasing appearance, and creates, in effect, many small areas of reflected glare which are emphasized in appearance by being adjacent the dark or black areas. Typically, this problem is addressed by manufacturers to produce the louver blades from a satin finish or semi diffuse aluminum. Such materials tend to smooth out or diffuse the localized patches of glare by their inherent properties of diffusion. While this is desirable from an appearance standpoint, the accuracy of the light control is reduced; and scattered light rays are introduced. The precision or the light pattern, therefore, is lost. The total elimination or brightness or glare at high angles disappears. In addition, the efficiency of the fixture is reduced by several percent, due to the multiple interreflections of the diffused portions of the light rays as they bounce between the louver blades forming each of the apertures.
An approach to solving at least one of these problems, the problem of a clear view of the interior of the fixture through the louvers at high angles of view, is addressed by the patent to Fain No. 4,630,181. Fain has a louver assembly which employs a translucent light-diffusing panel on top of the louvers. This panel is located between the louvers and the lamps within the fixture, and functions to provide standard diffusion of the light emanating from the fixture. Reduction in efficiency of the assembly, directly attributable to the translucent diffusing panel, is introduced; and because of the inherent characteristics of such a panel, increased scattering of light rays also occurs. The multiple interreflections of light rays passing through the diffuser panel and striking the louver blades also result in reduced efficiency. Some of the precision of the light pattern is lost; so that brightness elimination at high angles of view no longer occurs. Thus, while Fain attempts to solve one of the problems mentioned above concerning louver assemblies, the effort to solve that problem results in the additional disadvantages mentioned.
Two patents which are directed to variations of louver assemblies for the light-emitting opening of fluorescent lamp fixtures are the patents to Guth No. 3,093,323 and Cutler No. 3,152,277. The patent to Guth uses a prismatic light diffuser panel having longitudinally extending and transversely extending upwardly facing prisms, with smaller prisms formed in the surface of the diffuser which faces the interior of the room. No glare reducing louver assembly is suggested or disclosed in this patent.
The Cutler patent also is directed to a lens made in the form of individual glass lenses located in each of the louver apertures, to extend the image of the lamp through the louvers. Each of the lenses are the same throughout the fixture; and they are formed as an integral part of a specially constructed fluorescent lamp structure. The fixture of Cutler does not use standard fluorescent tubes, and is not designed to be used with such standard tubes.
The patent to Lewin No. 3,988,609 is directed to a high-efficiency wide light distribution panel. The panel includes three bands of prismatic elements, uniformly spaced across the panel an separated by substantially transparent bands with shallow flutes in them to soften the lamp images. Other prisms are included for increasing the spread of light to the sides of the fixture. This is not a fixture using a louver assembly; but it is one which is designed to even out the spread of light from the fixture over the area being illuminated.
It is desirable to provide a louver assembly for a light fixture which overcomes the disadvantages mentioned above with maximum efficiency, and which is aesthetically pleasing in appearance and easy to install.